The Palazzo Mocenigo detto "il Nero" is a palazzo on the Grand Canal in the sestiere of San Marco, Venice, Italy. Other Palazzi Mocenigo on each side include the Palazzo Mocenigo Casa Nuova and the Palazzo Mocenigo Casa Vecchia. The palazzo is located between the Rialto Bridge and St Mark's Square. It was occupied by the poet English Lord Byron (1788–1824) when he lived in Venice.

The Basilica di Santa Maria Gloriosa dei Frari, usually just called the Frari, is a church in Venice, northern Italy. One of the greatest churches in the city, it has the status of a minor basilica. It stands on the Campo dei Frari at the heart of the San Polo district. The church is dedicated to the Assumption (Italian: Assunzione della Beata Virgine).
https://en.wikipedia.org/wiki/Santa_Maria_Gloriosa_dei_Frari

Sam and companions merely passed through by train on their way to Venice, July 20, 1867.
Mark Twain Project: Quaker City Itinerary

Lake Como (Lago di Como in Italian, also known as Lario, after the Latin name of the lake; Lach de Comm in Insubric; Latin: Larius Lacus) is a lake of glacial origin in Lombardy, Italy. It has an area of 146 km², making it the third largest lake in Italy, after Lake Garda and Lake Maggiore. At over 400 m (1320 ft) deep, it is one of the deepest lakes in Europe, and the bottom of the lake is more than 200 metres (656 ft) below sea-level.

Twain made a day-trip from Como to Chiasso in nearby Switzerland, not mentioned in Innocents Abroad (Mark Twain A to Z)

The amphitheatre was built near the Porta Ticinese ("Ticino Gate") in the 2nd-3rd centuries AD, when Mediolanum grew as economical and political importance while Rome declined. It remained in use until the city was one of the capitals of the Western Roman Empire (4th or 5th centuries). Later it was abandoned after Christianism imposed the end to arena games, but also as, in the wake of the imperial crisis, animals to be used in the amphitheatre were no longer imported. It became a quarry for construction stones as early as the 4th century AD, when the Basilica of San Lorenzo was built.

I like to revel in the dryest details of the great cathedral. The building is five hundred feet long by one hundred and eighty wide, and the principal steeple is in the neighborhood of four hundred feet high. It has 7,148 marble statues, and will have upwards of three thousand more when it is finished. In addition it has one thousand five hundred bas-reliefs. It has one hundred and thirty-six spires—twenty-one more are to be added. Each spire is surmounted by a statue six and a half feet high.